1. Field of the Invention
This invention relates to an improved process for the selective separation of components of aqueous compositions by reverse osmosis or ultrafiltration. More particularly, this invention is directed to a process for obtaining membranes made of aromatic nitrogen-linked synthetic organic polymers.
2. Description of the Prior Art
Permeation selective (i.e., permselective) membranes which preferentially permeate certain components of liquid mixtures while retaining other components, have long been known, as has the principle of reverse osmosis, wherein a hydrostatic pressure in excess of the equilibrium osmotic pressure of a liquid mixture is applied to the mixture to force the more permeable components of the mixture, usually water, through the membrane in preference to the less permeable components, usually a salt, contrary to normal osmotic flow. Recent research in this field has been directed primarily toward the development of membranes for the reverse osmosis desalination of brackish and sea waters on a practical scale.
It is well known that complete separation of the more permeable from the less permeable components of liquid mixtures is never obtained with permselective membranes in practical use. All components of a mixture permeate to some degree through any membrane which has a practical permeation flux rate for the more permeable components. A principal goal with such membranes has been the production of membranes with economically attractive optimum balances of high flux rates for the more permeable components and high rejection efficiencies for the less permeable salt components of liquid mixtures. Aromatic nitrogen-linked synthetic organic polymers, such as those described in Richter and Hoehn U.S. Pat. No. 3,567,632, have been found useful in this respect, and continuing effort has been made to improve upon the balance of flux rate and the salt rejection properties of such membranes. For example, salt rejection is improved by contacting the membranes with hydrolyzable tannins as disclosed by Chen and Ganci U.S. Ser. No. 562,246 filed Mar. 26, 1975; or by contacting the membranes with a hydrous heavy metal composition as disclosed by Ganci U.S. Pat. No. 3,853,755; or by contacting the membranes with a selected ether as disclosed by Ganci, Jensen and Smith U.S. Pat. No. 3,808,303. As is seen, the effort has been directed toward enhancing the salt rejection properties by treatment of the membrane after it has been prepared.
In contrast, in the present invention, the water flux rate, rather than the salt rejection properties, has been improved over the rate of membranes not subjected to the treatment of this invention. In addition, in the present invention, the water flux rate is enhanced, not by treatment of the membrane, but rather by treatment during preparation of the membrane.